Optical measuring arrangements and methods of measuring a magnetic field utilizing the magneto-optic Faraday effect are known. The Faraday effect is defined as the rotation of the plane of polarization of linearly polarized light as a function of a magnetic field. The angle of rotation is proportional to the path integral over the magnetic field along the path traveled by the light with the Verdet constant as a proportionality constant. The Verdet constant depends in general on the material, the temperature, and the wavelength. To measure the magnetic field, a Faraday sensor device made of an optically transparent material such as glass is arranged in the magnetic field. The magnetic field causes the plane of polarization of linearly polarized light passed through the Faraday sensor device to rotate by an angle of rotation that can be analyzed for a measuring signal. Such magneto-optical measuring methods and arrangements are known for use in measuring electric currents. The Faraday sensor device is placed near a current conductor and detects the magnetic field generated by the current in the conductor. The Faraday sensor device generally surrounds the current conductor, so the measuring light travels around the current conductor in a closed path. In this case, the value of the angle of rotation is in good approximation directly proportional to the amplitude of the current to be measured. The Faraday sensor device may be designed as a solid glass ring around the current conductor or it may surround the current conductor in the form of a measuring winding consisting of an optical fiber (fiber coil) with at least one spire.
Advantages of these magneto-optical measuring arrangements and methods, in comparison with traditional inductive current transformers, include electrical isolation and insensitivity to electromagnetic disturbance. In the use of magneto-optic current transformers, however, problems are encountered due to the effects of mechanical vibrations on the sensor device and the optical leads, which can cause changes in intensity that falsify the measurement, as well as the effects of changes in temperature, for example, in the sensor device.
To reduce the effects of vibration on the measurement, it is known that two oppositely directed light signals, i.e., light signals propagating in opposite directions, can be transmitted through a Faraday sensor device. This known measure is based on the idea that the linear birefringences experienced by the two light signals along their common path due to vibrations as a reciprocal effect of the non-reciprocal Faraday effect can can be distinguished using suitable signal processing.
In a first known embodiment, two oppositely directed, linearly polarized light signals are transmitted through an optical fiber coil serving as a Faraday sensor device surrounding a current conductor. A twisted fiber or a spun hi-bi fiber (a high-birefringence fiber twisted during the drawing process) is provided as the optical fiber for the fiber coil. In addition to the Faraday effect, the optical fiber also has a circular birefringence that is high in comparison with the Faraday effect. After passing through the sensor device, each of the two light signals is broken down by a polarizing beam splitter into two components polarized normally to one another. A measuring signal corresponding essentially to the quotient of the Faraday measuring angle and the circular birefringence of the fiber, which is thus independent of the linear birefringence in the optical fiber, is derived by signal processing from a total of four light components. The resulting measuring signal is thus largely free of temperature-induced linear birefringence in the sensor device, but the measuring signal still depends on temperature because of the temperature-dependence of the circular birefringence of the fiber. In this known embodiment, the two oppositely directed light signals pass only through the Faraday sensor device along a common light path and are separated again by optical couplers on leaving the Faraday sensor device as shown in International.
In two other known embodiments, two light signals pass through an optical series connection consisting of a first optical fiber, a first polarizer, a Faraday sensor device, a second polarizer, and a second optical fiber in opposite directions. Each light signal is converted to an electric intensity signal by appropriate photoelectric transducers after passing through the optical series connection.
In the first embodiment known from U.S. Pat. No. 4,916,387, a solid glass ring surrounding the current conductor is provided as the Faraday sensor device. The polarization axes of the two polarizers are rotated by a 45.degree. angle to one another. For compensation of unwanted changes in intensity in the optical lead-in fibers with this measuring arrangement which is known from U.S. Pat. No. 4,916,387, it is assumed that the unwanted variations in intensity (noise) and the variations in intensity due to the Faraday effect are additively superimposed with different signs in the two electric intensity signals and thus can be separated. However, an indepth physical analysis leads to the result that mechanical movements of the two optical fibers for transmitting the two light signals essentially act as time-variable attenuation factors in the light intensities of the two light signals. U.S. Pat. No. 4,916,387 does not indicate how such different attenuation factors in the two optical fibers can be compensated.
In the second embodiment, which is known from the Journal of Lightwave Technology, vol. 12, no. 10, October 1994, pages 1882-1890, a fiber coil consisting of a single-mode fiber with a low birefringence is provided as the Faraday sensor device. The polarization axes of the two polarizers together form a polarizer angle that is different from 0.degree., preferably 45.degree.. Light from a single light source is split into two light signals and these signals are injected into the respective optical fiber through an optical coupler. A measuring signal corresponding to the quotient of the difference between the two intensity signals and the sum of the two intensity signals is derived from the two electric intensity signals that correspond to the light intensities of the respective light signals after passing through the series connection. Thus the attenuation factors of the two optical fibers can be essentially compensated. However, the light intensities of the two light signals must be set exactly the same when injected into the series connection.
Compensation of temperature effects on the measuring signal is not described in U.S. Pat. No. 4,916,387 or in the Journal of Lightwave Technology, vol. 12, no. 10, October 1994, pages 1882-1890. Instead, temperature-insensitive fiber coils are used as the sensor device. However, the manufacture of such fiber coils is comparatively problematical.